The present invention relates to the field of mobile telephone systems. A subscriber in a mobile telephone system, i.e. a subscriber station, for instance a radio telephone or a mobile station, may be registered in a radio network or a radio system, whereby it is connected to the radio network via system channels comprising control and traffic channels maintained by the base stations of the radio network.
In addition to the system channels provided in mobile phone systems, so-called direct mode channels can also be used in connection with a radio system, i.e. direct mode operation is applied. Subscriber stations using direct mode operation do not communicate directly with the radio network or its base stations. Direct mode channels are frequencies at which mobile phones or other means of communication are able to communicate directly with one another without the system, or via repeater stations, either with the base stations of the system or with other mobile stations.
Direct mode channels are typically used in situations where for instance a group of portable mobile phones are communicating with one another at such a long distance from the base station that system channels cannot be used.
Another important way of using direct mode channels is to increase the capacity when the traffic in the system increases rapidly, e.g., due to occurrence of an incident in some part of the service area of the system, for instance in some point-like part.
A direct mode channel is referred to with the terms direct or simplex channel, or a simplex connection. A direct mode channel is a channel which is typically not at all used by the system. It may be, for instance a channel with the same channel spacing as the channels of the system, for instance 12.5 kHz or 25 kHz. Among the mobile phones operating on a direct mode channel, the transmitting station has tuned its transmitter to the direct mode channel and transmits speech or data information. The other mobile phones set to direct mode operation have tuned their receivers to the same direct mode channel, whereby they are able to directly hear the transmission.
Operation on a direct mode channel may take place on an analog or digital modulation principle. A mobile phone transmitting on the channel may also transmit signalling information, such as information on rights of use and priorities or on a group operating on the channel, or data traffic. On the direct mode channel, an encryption may be carried out or plain speech can be transmitted.
Subscriber stations using direct mode operation communicate with other subscriber stations on a direct mode channel without being in direct contact with the base stations of the radio network. The subscriber stations using direct mode operation can, however, communicate with the radio network via repeater stations. A repeater station is a piece of equipment comprising two radio apparatuses connected to each other. The repeater station typically comprises two transceivers. The repeater station transmits the information messages transmitted on the direct mode channel to desired network elements, for instance to the base stations of the radio network, these base stations transmitting the information messages further to the switching centers of the radio network. The information messages forwarded by the repeater station may be, for instance speech or data or signalling messages. In using a repeater, the traffic on the direct mode channel takes place, for instance on a semiduplex principle.
It should be noticed that, in direct mode operation, repeaters/repeater stations may also forward traffic between two mobile stations located within the coverage area of the repeater concerned, the mobile stations not being necessarily located within the coverage area of each other. A repeater may also act as a gateway between a mobile communication system, especially a base station thereof, and mobile stations communicating on the direct mode channel.
In prior art solutions, mobile stations communicating on a direct mode channel communicate on the direct mode channel in every situation in such a manner that the transmissions of the mobile stations are synchronized to be simultaneous with the downlink transmissions on a control channel maintained by a base station. A solution of the prior art causes delays of different lengths to signalling and speech messages transmitted between the direct mode channel and the system channel. The delays are naturally undesirable when considering the speed requirements of the connections to be established, and the delays also deteriorate the quality of the speech to be forwarded. In addition, the control of such delays renders a repeater station more complicated, and worse as regards the properties thereof, since long-term buffering of the transmitted information is required. An especially harmful feature is that call set-up times are prolonged as a result of the delays. The call set-up times are crucial, for instance in PMR mobile telephone systems (PMR=Private Mobile Radio), the users of which probably consist of authorities engaged in emergency duties, who require fast call set-up.